NDRG2 is a novel p53-associated regulator of apoptosis in C6-originated astrocytes exposed to oxygen-glucose deprivation

N-myc Downstream-Regulated Gene 2 (NDRG2) Function as a Positive Regulator of Apoptosis: A New Insight into NDRG2 as a Tumor Suppressor

N-myc downstream-regulated gene 2 (NDRG2) is a tumor suppressor gene that increases tumor sensitivity to anticancer drugs, slows tumor progression, and inhibits metastasis. NDRG2 is suppressed in various aggressive tumor positions, whereas NDRG2 expression is associated with patient prognosis, such as an improved survival rate. In this review, we summarize the tumor suppressor mechanism of NDRG2 and provide information on the function of NDRG2 concerning the susceptibility of cells to apoptosis. NDRG2 increases the susceptibility to apoptosis in various physiological environments of cells, such as development, hypoxia, nutrient deprivation, and cancer drug treatment. Although the molecular and cell biological mechanisms of NDRG2 have not been fully elucidated, we provide information on the mechanisms of NDRG2 in relation to apoptosis in various environments. This review can assist the design of research regarding NDRG2 function and suggests the potential of NDRG2 as a molecular target for cancer patients.

Upregulation of P53 promotes nucleus pulposus cell apoptosis in intervertebral disc degeneration through upregulating NDRG2

P53 is an apoptosis marker which is involved in determining nucleus pulposus (NP) cell fate. Little is known about P53 interaction with N-Myc downstream-regulated gene 2 (NDRG2) in intervertebral disc degeneration (IVDD). Here, we studied the role of the P53-NDRG2 axis in IVDD. We found that NDRG2 was expressed in NP tissue obtained from patients with IVDD. The level of NDRG2 was positively related to the severity of IVDD, as determined by Pfirrmann grading. Subsequently, we overexpressed NDRG2 in human NP cells by adenoviral transfection and studied the effects of increased levels of NDRG2 on the viability and apoptosis of these cells. NDRG2 overexpression induced NP cell apoptosis and reduced viability in NP cells obtained from patient with IVDD. We also found that the level of P53 was elevated in NP cells from patients with IVDD and treatment with exogenous P53 upregulated NDRG2 in NP cells. Last, IVDD model was established in P53 knockout mice and the pathological changes in the intervertebral discs and NDRG2 expression were examined. P53 knockout can reduce the damage of NP tissues after IVDD surgery to some extent. Restoration of NDRG2 antagonized the effect of P53 knockout on IVDD. Collectively, this study suggests that elevated P53 in NP cells stimulates apoptosis of the cells by upregulating NDRG2 expression, thereby exacerbating IVDD.

NDRG2 is expressed on enteric glia and altered in conditions of inflammation and oxygen glucose deprivation/reoxygenation

Enteric glial cells are more abundant than neurons in the enteric nervous system. Accumulating evidence has demonstrated that enteric glial cells share many properties with astrocytes and play pivotal roles in intestinal diseases. NDRG2 is specifically expressed in astrocytes and is involved in various diseases in the central nervous system. However, no studies have demonstrated the expression of NDRG2 in enteric glial cells. We performed immunostaining of adult mouse tissue, human colon sections, and primary enteric glial cells and the results showed that NDRG2 was widely expressed in enteric glial cells. Meanwhile, our results showed that NDRG2 was upregulated after treatment with pro-inflammatory cytokines and exposure to oxygen glucose deprivation/reoxygenation, indicating that NDRG2 might be involved in these conditions. Moreover, we determined that NDRG2 translocated to the nucleus after treatment with pro-inflammatory cytokines but not after exposure to oxygen glucose deprivation/reoxygenation. This study is the first to show the expression and distribution of NDRG2 in the enteric glia. Our results indicate that NDRG2 might be involved in the pathogenesis of enteric inflammation and ischemia/reperfusion injury. This study shows that NDRG2 might be a molecular target for enteric nervous system diseases.

Astrocyte-specific NDRG2 gene: functions in the brain and neurological diseases

In recent years, the roles of astrocytes of the central nervous system in brain function and neurological disease have drawn increasing attention. As a member of the N-myc downstream-regulated gene (NDRG) family, NDRG2 is principally expressed in astrocytes of the central nervous system. NDRG2, which is involved in cell proliferation and differentiation, is commonly regarded as a tumor suppressor. In astrocytes, NDRG2 affects the regulation of apoptosis, astrogliosis, blood-brain barrier integrity, and glutamate clearance. Several preclinical studies have revealed that NDRG2 is implicated in the pathogenesis of many neurological diseases not limited to tumors (mostly glioma in the nervous system), such as stroke, neurodegeneration (Alzheimer's disease and Parkinson's disease), and psychiatric disorders (depression and attention deficit hyperactivity disorder). This review summarizes the biological functions of NDRG2 under physiological and pathological conditions, and further discusses the roles of NDRG2 during the occurrence and development of neurological diseases.

NDRG2 Expression Correlates with Neurofibrillary Tangles and Microglial Pathology in the Ageing Brain

Astrocytes play a major role in the pathogenesis of a range of neurodegenerative diseases, including Alzheimer’s disease (AD), undergoing dramatic morphological and molecular changes that can cause potentially both beneficial and detrimental effects. They comprise a heterogeneous population, requiring a panel of specific phenotype markers to identify astrocyte subtypes, changes in function and their relation to pathology. This study aimed to characterise expression of the astrocyte marker N-myc downstream regulated gene 2 (NDRG2) in the ageing brain, investigate the relationship between NDRG2 and a panel of astrocyte markers, and relate NDRG2 expression to pathology. NDRG2 specifically immunolabelled the cell body and radiating processes of astrocytes in the temporal cortex of the Cognitive Function and Ageing Study (CFAS) neuropathology cohort. Expression of NDRG2 did not correlate with other astrocyte markers, including glial fibrillary acidic protein (GFAP), excitatory amino acid transporter 2 (EAAT2) and glutamine synthetase (GS). NDRG2 showed a relationship to AT8⁺ neurofibrillary tangles (p = 0.001) and CD68⁺ microglia (p = 0.047), but not β-amyloid plaques or astrocyte nuclear γH2AX immunoreactivity, a marker of DNA damage response. These findings provide new insight into the astrocyte response to pathology in the ageing brain, and suggest NDRG2 may be a potential target to modulate this response.

Nervous NDRGs: the N-myc downstream-regulated gene family in the central and peripheral nervous system

The N-Myc downstream-regulated gene (NDRG) family consists of four members (NDRG1, NDRG2, NDRG3, NDRG4) that are differentially expressed in various organs and function in important processes, like cell proliferation and differentiation. In the last couple of decades, interest in this family has risen due to its connection with several disorders of the nervous system including Charcot-Marie-Tooth disease and dementia, as well as nervous system cancers. By combining a literature review with in silico data analysis of publicly available datasets, such as the Mouse Brain Atlas, BrainSpan, the Genotype-Tissue Expression (GTEx) project, and Gene Expression Omnibus (GEO) datasets, this review summarizes the expression and functions of the NDRG family in the healthy and diseased nervous system. We here show that the NDRGs have a differential, relatively cell type-specific, expression pattern in the nervous system. Even though NDRGs share functionalities, like a role in vesicle trafficking, stress response, and neurite outgrowth, other functionalities seem to be unique to a specific member, e.g., the role of NDRG1 in myelination. Furthermore, mutations, phosphorylation, or changes in expression of NDRGs are related to nervous system diseases, including peripheral neuropathy and different forms of dementia. Moreover, NDRG1, NDRG2, and NDRG4 are all involved in cancers of the nervous system, such as glioma, neuroblastoma, or meningioma. All in all, our review elucidates that although the NDRGs belong to the same gene family and share some functional features, they should be considered unique in their expression patterns and functional importance for nervous system development and neuronal diseases.

NDRG4 in gastric cancer determines tumor cell proliferation and clinical outcome

As a novel candidate tumor suppressor, NDRG4 is largely unstudied in human malignancies. In this study, we investigated the protein expression level of NDRG4 in gastric cancer and its association with outcome of patients. In the present study, we recruited 286 patients with gastric cancer and investigated the protein and mRNA expression of NDRG4 in cancer and adjacent normal specimens by immunohistochemistry assay and real-time PCR. The association of NDRG4 level with clinicopathological characteristics was investigated by appropriate statistical analysis. NDRG4 overexpression and knockdown cell lines were established in order to detect its impact on proliferation and apoptosis. Significant decreased protein and mRNA expression of NDRG4 was found in gastric cancer, compared with adjacent normal specimens. Besides, it was found that NDRG4 protein expression in gastric cancer was significantly associated with tumor differentiation, invasion, metastasis, and stage. Patients with tumors of decreased NDRG4 level were more likely to have unfavorable disease-free and overall survival, in both univariate and multivariate analysis. In addition, overexpression of NDRG4 suppressed cell proliferation of gastric cancer cells in vitro; conversely, the proliferation of gastric cancer cells were enhanced by knockdown of NDRG4. These results proved for the first time that NDRG4 could be a potential tumor suppressor and prognostic marker of gastric cancer.

NDRG2 contributes to cisplatin sensitivity through modulation of BAK-to-Mcl-1 ratio

The downregulation of N-Myc downstream-regulated gene 2 (NDRG2) is known to be associated with the progression and poor prognosis of several cancers. Sensitivity to anti-cancer may be associated with a good prognosis in cancer patients, and NDRG2, which is induced by p53, sensitizes the cells to chemotherapy. However, the unique function of NDRG2 as an inducer of apoptosis under chemotreatment has not been sufficiently studied. In this study, we investigated the role of NDRG2 in chemo-sensitivity, focusing on cisplatin in U937 histiocytic lymphoma, which has the loss-of-functional mutation in p53. NDRG2 promoted the sensitivity to cisplatin through the modulation of the BAK-to-Mcl-1 ratio. The degradation of Mcl-1 and increase in BAK were mediated by JNK activation and the eIF2α/p-eIF2α pathway, respectively, which depended on PKR activation in NDRG2-overexpressed U937 (U937-NDRG2) cells. NOX5 was highly expressed in U937-NDRG2 cells and contributed to ROS production after cisplatin treatment. ROS scavenging or NOX5-knockdown successfully inhibited the sensitivity of U937-NDRG2 cells to cisplatin. Taken together, these findings indicate that NDRG2 contributed to the increased sensitivity to ciplatin through the modulation of Bak-to-Mcl-1 ratio regulated by NOX5-ROS-PKR pathway; therefore, we suggest that NDRG2 may be a molecular target for improving the efficacy of drug treatment in cancer patients.

Protective Effects of Fetal Zone Steroids Are Comparable to Estradiol in Hyperoxia-Induced Cell Death of Immature Glia

Impaired neurodevelopment in preterm infants is caused by prematurity itself; however, hypoxia/ischemia, inflammation, and hyperoxia contribute to the extent of impairment. Because preterm birth is accompanied by a dramatic decrease in 17β-estradiol (E2) and progesterone, preliminary clinical studies have been carried out to substitute these steroids in preterm infants; however, they failed to confirm significantly improved neurologic outcomes. We therefore hypothesized that the persistently high postnatal production of fetal zone steroids [mainly dehydroepiandrosterone (DHEA)] until term could interfere with E2-mediated protection. We investigated whether E2 could reduce hyperoxia-mediated apoptosis in three immature glial cell types and detected the involved receptors. Thereafter, we investigated protection by the fetal zone steroids DHEA, 16α-hydroxy-DHEA, and androstenediol. For DHEA, the involved receptors were evaluated. We examined aromatases, which convert fetal zone steroids into more estrogenic compounds. Finally, cotreatment was compared against single hormone treatment to investigate synergism. In all cell types, E2 and fetal zone steroids resulted in significant dose-dependent protection, whereas the mediating receptors differed. The neuroprotection by fetal zone steroids highly depended on the cell type-specific expression of aromatases, the receptor repertoire, and the potency of the fetal zone steroids toward these receptors. No synergism in fetal zone steroid and E2 cotreatment was detected in two of three cell types. Therefore, E2 supplementation may not be beneficial with respect to neuroprotection because fetal zone steroids circulate in persistently high concentrations until term in preterm infants. Hence, a refined experimental model for preterm infants is required to investigate potential treatments.

MALDI-Imaging Mass Spectrometry: a step forward in the anatomopathological characterization of stenotic aortic valve tissue

Aortic stenosis (AS) is the most common form of valve disease. Once symptoms develop, there is an inexorable deterioration with a poor prognosis; currently there are no therapies capable of modifying disease progression, and aortic valve replacement is the only available treatment. Our goal is to study the progression of calcification by matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI-IMS) and get new insights at molecular level that could help in the understanding of this disease. In this work, we analyzed consecutive slices from aortic valve tissue by MALDI-IMS, to establish the spatial distribution of proteins and peptides directly from the surface of the histological sections. The analysis showed different structures corresponding to regions observed in conventional histology, including large calcification areas and zones rich in collagen and elastic fibers. Peptide extraction from the tissue, followed by liquid chromatography mass spectrometry analysis, provided the identification of collagen VI α-3 and NDRG2 proteins which correlated with the masses obtained by MALDI-IMS and were confirmed by immunohistochemistry. These results highlighted the molecular mechanism implied in AS using MALDI-IMS, a novel technique never used before in this pathology. In addition, we can define specific regions proving a complementary resolution of the molecular histology.

NDRG4 is a novel oncogenic protein and p53 associated regulator of apoptosis in malignant meningioma cells

Aggressive meningiomas exhibit high levels of recurrence, morbidity and mortality. When surgical and radiation options are exhausted, there is need for novel molecularly-targeted therapies. We have recently identified NDRG4 overexpression in aggressive meningiomas. NDRG4 is a member of the N-Myc Downstream Regulated Gene (NDRG) family of the alpha/beta hydrolase superfamily. We have demonstrated that NDRG4 downregulation results in decreased cell proliferation, migration and invasion. In follow up to our prior studies; here we demonstrate that the predominant form of cell death following NDRG4 silencing is apoptosis, utilizing Annexin-V flow cytometry assay. We show that apoptosis caused by p53 upregulation, phosphorylation at Ser15, BAX activation, Bcl-2 and BcL-xL downregulation, mitochondrial cytochrome c release and execution of caspases following NDRG4 depletion. Sub-cellular distribution of BAX and cytochrome c indicated mitochondrial-mediated apoptosis. In addition, we carried out the fluorescence cytochemical analysis to confirm mitochondrial-mediated apoptosis by changes in mitochondrial membrane potential (Ψm), using JC-1 dye. Immunoprecipitation and immunofluorescence confirmed binding of NDRG4 to p53. In addition, we demonstrate that apoptosis is mitochondrial and p53 dependent. The proapoptotic effect of p53 was verified by the results in which a small molecule compound PFT-α, an inhibitor of p53 phosphorylation, is greatly protected against targeting NDRG4 induced apoptosis. These findings bring novel insight to the roles of NDRG4 in meningioma progression. A better understanding of this pathway and its role in meningioma carcinogenesis and cell biology is promising for the development of novel therapeutic targets for the management of aggressive meningiomas.

2-Arachidonylglycerol Protects Primary Astrocytes Exposed to Oxygen-Glucose Deprivation Through a Blockade of NDRG2 Signaling and STAT3 Phosphorylation

The human N-Myc downstream-regulated gene 2 (NDRG2) is expressed in astrocytes, and may be involved in the modulation of gliacyte function in the central nervous system. Our previous study found suppression of NDRG2 up-regulation in reactive astrocytes in cerebral ischemic tolerance. 2-Arachidonylglycerol (2-AG) can induce cerebral ischemic tolerance. However, the underlying mechanism of NDRG2 in cytoprotection induced by 2-AG in primary astrocytesis still unknown. In this study, we investigated the role of NDRG2 in cerebral ischemic tolerance induced by 2-AG after oxygen-glucose deprivation (OGD) in primary astrocytes. The results showed that primary astrocytes exposed to OGD resulted in marked increase of lactate dehydrogenase (LDH) release and decrease of methyl thiazolyl tetrazolium (MTT) reduction activity in comparison to control cultures. The levels of NDRG2 and phospho-signal transducer and activator of transcription 3 (pSTAT3) in the OGD group were comparably higher than those in the control group, and the up-regulation of NDRG2 and pSTAT3 was suppressed in NDRG2 siRNA group. The cell viability in the 2-AG group was higher than that in the OGD group, and transfecting the NDRG2 pSRL-CDH1-GFP vector reversed the protective effects of 2-AG. The levels of NDRG2 and pSTAT3 in the 2-AG group were lower than those in the OGD group. 2-AG suppressed STAT3 phosphorylation by decreased expression of NDRG2. In conclusion, 2-AG protects primary astrocytes exposed to oxygen-glucose deprivation through a blockade of NDRG2 signaling and STAT3 phosphorylation. These findings bring insight to the roles of NDRG2 in ischemic-hypoxic injury and provide novel potential targets for future potent clinical therapies on cerebral ischemia injury.

NDRG2 phosphorylation provides negative feedback for SGK1-dependent regulation of a kainate receptor in astrocytes

Glutamate receptors play an important role in the function of astrocytes. Among their tasks is the regulation of gliotransmission, gene expression and exocytosis of the tissue-type plasminogen activator (tPA), which has an enhancing effect on N-methyl-D-aspartate (NMDA) receptors and thus prevent over-excitation of neighboring neurons. The kainate receptor GluK2, which is expressed in neurons and astrocytes, is under tight regulation of the PI3-kinase SGK pathway as shown in neurons. SGK1 targets include N-myc downstream-regulated genes (NDRGs) 1 and 2 (NDRG1, NDRG2), proteins with elusive function. In the present study, we analyzed the effects of SGK1, NDRG1, and NDRG2 on GluK2 current amplitude and plasma membrane localization in astrocytes and heterologous expression. We demonstrate that NDRG1 and NDRG2 themselves have no effect on GluK2 current amplitudes in heterologous expressed ion channels. However, when NDRG2 is coexpressed with GluK2 and SGK1, the stimulating effect of SGK1 on GluK2 is suppressed both in heterologous expression and in astrocytes. Here, we reveal a new negative feedback mechanism, whereby GluK2 stimulation by SGK1 is regulated by parallel phosphorylation of NDRG2. This regulation of GluK2 by SGK1 and NDRG2 in astrocytes may play an important role in gliotransmission, modulation of gene expression and regulation of exocytosis of tPA.

N-myc downstream-regulated gene 2 in the nervous system: from expression pattern to function

Human N-myc downstream-regulated gene 2 (NDRG2) has been shown to be a multifunctional protein associated with cell proliferation, differentiation, transmembrane transport, and stress responses. In most mammalian brains, NDRG2 is principally expressed in astrocytic cells throughout different regions. NDRG2 has been increasingly implicated in the regulation of neurogenesis and in the development of nervous system diseases, including neurodegeneration, ischemia, and glioblastoma. This review summarizes the distribution and subcellular localization of NDRG2 in brain tissues, highlights the physiological actions of NDRG2 in the nervous system, and further discusses the roles of NDRG2 during the occurrence and development of several nervous system diseases.

Inhibition of N-myc Downstream–regulated Gene-2 Is Involved in an Astrocyte-specific Neuroprotection Induced by Sevoflurane Preconditioning

Background:

Mechanism of sevoflurane preconditioning–induced cerebral ischemic tolerance is unclear. This study investigates the role of N-myc downstream–regulated gene-2 (NDRG2) in the neuroprotection of sevoflurane preconditioning in ischemic model both in vivo and in vitro.

Methods:

At 2 h after sevoflurane (2%) preconditioning for 1 h, rats were subjected to middle cerebral artery occlusion for 120 min. Neurobehavioral scores (n = 10), infarct volumes (n = 10), cellular apoptosis (n = 6), and NDRG2 expression (n = 6) were determined at 24 h after reperfusion. In vitro, cultural astrocytes were exposed to oxygen–glucose deprivation for 4 h. Cellular viability, cytotoxicity, apoptosis, and NDRG2 expression (n = 6) were evaluated in the presence or absence of NDRG2-specific small interfering RNA or NDRG2 overexpression plasmid.

Results:

Sevoflurane preconditioning decreased apoptosis (terminal deoxynucleotidyl transferase–mediated 2’-deoxyuridine 5’-triphosphate nick-end labeling–positive cells reduced to 31.2 ± 5.3% and cleaved Caspase-3 reduced to 1.42 ± 0.21 fold) and inhibited NDRG2 expression (1.28 ± 0.15 fold) and nuclear translocation (2.21 ± 0.29 fold) in ischemic penumbra. Similar effects were observed in cultural astrocytes exposed to oxygen–glucose deprivation. NDRG2 knockdown by small interfering RNA attenuated oxygen–glucose deprivation–induced injury (cell viability increased to 80.5 ± 4.1%; lactate dehydrogenase release reduced to 30.5 ± 4.0%) and cellular apoptosis (cleaved Caspase-3 reduced to 1.55 ± 0.21 fold; terminal deoxynucleotidyl transferase–mediated 2’-deoxyuridine 5’-triphosphate nick-end labeling–positive cells reduced to 18.2 ± 4.3%), whereas NDRG2 overexpression reversed the protective effects of sevoflurane preconditioning. All the data are presented as mean ± SD.

Conclusion:

Sevoflurane preconditioning inhibits NDRG2 up-regulation and nuclear translocation in astrocytes to induce cerebral ischemic tolerance via antiapoptosis, which represents one new mechanism of sevoflurane preconditioning and provides a novel target for neuroprotection.

NDRG2 overexpression enhances glucose deprivation-mediated apoptosis in breast cancer cells via inhibition of the LKB1-AMPK pathway

The newly identified tumor suppressor, N-myc downstream-regulated gene 2 (NDRG2), has been studied in various cancers because of its anticancer and antimetastasis effects. In this study, we examined the effect of NDRG2 expression on cell viability in MDA-MB-231 human breast cancer cells under conditions that are similar to the microenvironment of solid tumors, which include glucose deprivation. NDRG2 overexpression enhanced the pro-apoptotic effects of glucose deprivation. Glucose deprivation also induced the activation of AMP-activated protein kinase (AMPK), which plays a role in protecting tumor cells from metabolic stresses. NDRG2 overexpression strongly reduced glucose deprivation-induced AMPK phosphorylation and increased the cleavage of poly (ADP-ribose) polymerase (PARP), which indicated the induction of apoptosis. The expression of a constitutively active form of AMPK effectively blocked glucose deprivation-induced apoptosis in NDRG2-overexpressing MDA-MB-231 cells. Moreover, NDRG2 overexpression also enhanced the pro-apoptotic effects of 2-deoxyglucose (2-DG) or hypoxia, an inducer of metabolic stresses. Finally, we showed that LKB1 is an upstream kinase of AMPK that is involved in the inhibition of glucose deprivation-induced AMPK activity in NDRG2-overexpressing cells. Our findings collectively suggest that NDRG2 is a negative regulator of AMPK activity and functions as a sensitizer of glucose deprivation.

Hyperglycemia-induced oxidative stress induces apoptosis by inhibiting PI3-kinase/Akt and ERK1/2 MAPK mediated signaling pathway causing downregulation of 8-oxoG-DNA glycosylase levels in glial cells

Glial cells are very important for normal brain function and alterations in their activity due to hyperglycemia, could contribute to diabetes-related cognitive dysfunction. Oxidative insults often cause rapid changes in almost all cells including glial cells. However, pathophysiologic mechanisms that lead to diabetic complications are not completely elucidated. Therefore, we examined whether elevated glucose levels directly or indirectly disrupt antioxidant defense mechanisms causing alterations in signaling pathways, cell cycle dysregulation, and reactive oxygen/nitrogen species-mediated apoptosis in glial cells. Findings of this study demonstrated that exposure of glial cells to high glucose markedly induces cellular and molecular injuries, as evidenced by elevated levels of reactive oxygen/nitrogen species, biomolecules damage, cell cycle dysregulation, decrease in antioxidant enzymes, and decrease in cell viability. Pretreatment of cells with N-acetyl-L-cysteine reduced high glucose-induced cytotoxicity by increasing the levels of antioxidant enzymes, and decreasing the number of apoptotic cells. Further, at molecular level high glucose treatment resulted in a significant increase in phosphorylation of Akt, MAPKs, tuberin, down regulation of 8-oxoG-DNA glycosylase and increase in 8-hydroxydeoxyguanosine accumulations. Pretreatment of cells with N-acetyl-L-cysteine, phosphatidylinositol3-kinase/Akt and ERK1/2 inhibitors completely abolished the apoptotic effects of high glucose. Moreover, N-acetyl-L-cysteine significantly inhibited reactive oxygen/nitrogen species generation, elevated antioxidants levels, inhibited Akt, ERK1/2, tuberin phosphorylation, decreased 8-hydroxydeoxyguanosine accumulation and upregulated 8-oxoG-DNA glycosylase expression. Our results demonstrate that high glucose induces apoptosis and inhibits proliferation of glial cells, which may be mediated by the phosphorylation of tuberin, down regulation of 8-oxoG-DNA glycosylase and 8-hydroxydeoxyguanosine accumulation via activation of Akt and ERK1/2MAPK pathways.

Estrogen regulates the expression of Ndrg2 in astrocytes

N-myc downstream-regulated gene 2 (Ndrg2) is a newly identified molecule that is mainly expressed in astrocytes within the central nervous system (CNS) and is involved in the proliferation and activation of astrocytes. 17β-estradiol (E2) is one of the most important circulating hormones, and in the CNS, astrocytes are a target and potential mediator of the action of E2. Our most recent study found that DPN, an estrogen receptor (ER) β-specific agonist, activated the Ndrg2 promoter and elevated endogenous NDRG2 protein expression in MCF7, HSG and T-47D cells. However, whether E2 regulates Ndrg2 expression in astrocytes remains unknown. Here, we conducted both in vivo and in vitro experiments and found that ERβ co-localized with NDRG2 in astrocytes. Furthermore, in primary cultured astrocytes, we demonstrated that E2 up-regulated Ndrg2 mRNA and protein expression in a dose- and time-dependent manner and that the ERβ agonist DPN but not the ERα agonist PPT up-regulated Ndrg2 expression. In vivo, we found that in the hippocampus of adult ovariectomized (OVX) female mice, Ndrg2 mRNA and protein expression were significantly decreased compared with those in normal adult female mice. After the OVX mice received continuous subcutaneous injections of 50μg/kg E2, 100μg/kg E2 or the ERβ agonist DPN for 10 days, the Ndrg2 expression significantly increased compared with that of the OVX mice. Our results indicate that E2 may affect astrocytes by regulating Ndrg2 expression.

Gliopreventive effects of guanosine against glucose deprivation in vitro

Guanosine, a guanine-based purine, is recognized as an extracellular signaling molecule that is released from astrocytes and confers neuroprotective effects in several in vivo and in vitro studies. Astrocytes regulate glucose metabolism, glutamate transport, and defense mechanism against oxidative stress. C6 astroglial cells are widely used as an astrocyte-like cell line to study the astrocytic function and signaling pathways. Our previous studies showed that guanosine modulates the glutamate uptake activity, thus avoiding glutamatergic excitotoxicity and protecting neural cells. The goal of this study was to determine the gliopreventive effects of guanosine against glucose deprivation in vitro in cultured C6 cells. Glucose deprivation induced cytotoxicity, an increase in reactive oxygen and nitrogen species (ROS/RNS) levels and lipid peroxidation as well as affected the metabolism of glutamate, which may impair important astrocytic functions. Guanosine prevented glucose deprivation-induced toxicity in C6 cells by modulating oxidative and nitrosative stress and glial responses, such as the glutamate uptake, the glutamine synthetase activity, and the glutathione levels. Glucose deprivation decreased the level of EAAC1, the main glutamate transporter present in C6 cells. Guanosine also prevented this effect, most likely through PKC, PI3K, p38 MAPK, and ERK signaling pathways. Taken together, these results show that guanosine may represent an important mechanism for protection of glial cells against glucose deprivation. Additionally, this study contributes to a more thorough understanding of the glial- and redox-related protective properties of guanosine in astroglial cells.